A photoelectric conversion element capable of converting light into electric energy can be utilized for solar cells, an image sensor, or the like. In particular, an image sensor for reading out the current generated from incident light in a photoelectric conversion element by a CCD or CMOS circuit is widely used.
Conventionally, in an image sensor using a photoelectric conversion element, an inorganic material has been used as a material for forming a photoelectric conversion film. However, since the inorganic material has low color selectivity, it has been required to selectively transmit each of the red, green and blue colors to incident light using a color filter, and to absorb the respective lights by a photoelectric conversion film. However, when a color filter is used, the pitch of an object interferes with the pitch of an image element when a fine object is taken, and an image different from the original image is generated (this is referred to as a moire defect). In order to suppress the defect, an optical lens or the like is required, but there is a disadvantage that the light utilization efficiency and the aperture ratio are lowered by the color filter and the optical lens.
On the other hand, in recent years, there has been an increasing demand for high resolution of an image sensor, and the fineness of the pixels has been advanced. Accordingly, although the size of the pixel becomes smaller, as a result of decreasing the size of the pixel, the amount of the light reaching the photoelectric conversion element of each pixel is decreased, and therefore, reduction in the sensitivity becomes a problem.
In order to solve this problem, researches on the photoelectric conversion element using an organic compound have been made. Since an organic compound can selectively absorb the light in a specific wavelength region out of the incident light by designing the molecular structure, a color filter is not required. Further, the absorption coefficient is high, and therefore, the light utilization efficiency can be increased.
As a photoelectric conversion element using an organic compound, specifically, an element having a constitution in which a pn junction structure or a bulk heterojunction structure is introduced into a photoelectric conversion film sandwiched between both electrodes is known (for example, see PTLs 1 to 3). In addition, in order to decrease dark current, an element having a constitution in which a charge blocking layer is inserted is also known (for example, see PTL 4).